Improvement of thermophilic anaerobic digestion stability by trace elements augmentation: preliminary results.

Thermophilic anaerobic digestion is a suitable technology to treat agricultural waste because of its higher biogas production, hygienisation effect and solids removal efficiency. Although these benefits, poorer effluent quality and instability are encountered in some cases (De Baere, 2000; van Lier et al., 1997). The anaerobic co-digestion of winery waste at 55°C, operating at 23 days HRT and organic loading rate of 3.1 kg COD/m3d, was characterized by accumulation of volatile fatty acids, pH fall and reduction of biogas production, while mesophilic process was steady at long term. The study evaluated the effect of trace elements (iron, cobalt and nickel) augmentation in the thermophilic reactor at concentration of 4.3 mg Fe /l, 0.46 mg Ni/l and 0.51 mg Co/l. The addition improved the process stability: pH became constant and average volatile fatty acids concentration was 320 mgCOD/l. The biogas production increased from 0.38 to 0. 45 m3/kg COD, corresponding with COD removal of 90%, while mesophilic reactor removed the 78% of total COD. The higher solids removal (28%) allowed to concentrate the phosphorus in the particulate fraction and transformed nitrogen into more available form for plants growth, then digestate had interesting characteristics as fertilizer.